In liquid crystal display devices, it is necessary to conduct an alternating current drive in which the polarity of a voltage applied to a liquid crystal is alternately changed for every frame. The reason resides in that continuous application of a voltage having a DC (direct current) component to a liquid crystal causes impurity-ions contained in the liquid crystal material at a very small amount are gathered around an electrode, thereby making it difficult to apply a correct voltage upon the liquid crystal. Therefore, conventional active matrix type liquid crystal display devices use the driving method as follows:
A gate line inversion driving method for alternately changing the polarity of a signal which is written to pixels with respect to a common electrode for every pixel row;
A data line inversion driving method for alternately changing the polarity of a signal for every pixel column; or
A dot inversion driving method for changing the polarity of a signal in a pixel unit in a checkered pattern manner.
These methods do not only achieve the above-mentioned alternating current drive, but also are advantageous to reduce flickering. The reason resides in that these methods spaciously average slight differences between the transmittances when signals having positive and negative polarities are written in pixels, so that flicker which perceived by human eyes can be reduced.
One of causes of the transmittance difference which is generated even when positive and negative polarity signals are written in pixels is a variation of a voltage applied to the pixels due to leakage current from a pixel transistor. It is a liquid crystal display device used for a projector that is most influenced by this problem. The reason is that it is necessary to irradiate the liquid crystal with very strong light in order to project a bright image from the projector, so that photo-leakage current from the pixel transistor becomes larger.
One of the performances required for the liquid crystal devices for the projector is high transmittance. It is necessary to provide a high aperture ratio which is a ratio of an area which transmits light with respect to the whole area of each pixel in order to increase the transmittance. In order to increase the aperture ratio, it is necessary to reduce an area on which wiring, pixel transistor and storage capacitor are formed and to decrease an area on which the orientation of molecules of liquid crystal is disturbed at the edge of a pixel electrode. The lowering of the aperture ratio due to orientation disturbance of liquid crystal molecules becomes serious particularly in such a high-resolution liquid crystal display device having a pixel pitch of 20 μm or less. The orientation disturbance of the liquid crystal molecules occurs due to the fact that the liquid crystal molecules have a tendency to orient along a lateral electric field due to the potential difference between adjacent pixel electrodes. The orientation disturbance is most remarkable if signals having different polarities are written into adjacent pixels. As a technique for preventing the orientation disturbance from occurring, there has been a used a frame inversion driving method in which signals having same polarity are written into all of pixels of the liquid crystal display device. However, this frame inversion driving method has a problem that the flicker becomes larger.
A technique to avoid this problem is described in Patent Document 1 described below. A method disclosed in Patent Document 1 reduces screen flicker by shortening the frame period during which signals of one screen are written into the liquid crystal display device even if the frame inversion driving method is used. In this method, a first video signal having the same polarity is applied to a plurality of pixel electrodes via a plurality of source signal lines (data signal lines) in a first frame period and then a second video signal having the polarity opposite to that of the first video signal is applied to a plurality of pixel electrodes via a plurality of source signal lines (data signal lines) in a frame period subsequent to the first frame period. The first and second frame period is set to 8.3 ms or less. In such a manner, this method reduces the voltage variation ascribable to leakage current from a pixel transistor by driving the liquid crystal display device at a frame frequency of 120 Hz or higher, which is not less than double the conventional frequency. This method utilizes a fact that flickering is hard to perceive for human eyes when the screen is rewritten at a high speed.
[Patent Document 1]
JP-P2001-92426A (page 5, 6 and FIG. 1)